The present invention relates to a method for the measurement of stresses exerted on a mechanical part and a method for the secure fastening of this device. It can be applied notably to the measurement of the stresses exerted on or the strains undergone by mechanical parts such as brake jaws or brake calipers, or traction elements for example. More generally, it can be applied to the measurement of stresses exerted by all types of mechanical parts, whatever their function, the invention being applicable also to heavy and bulky parts that are difficult to handle or are designed to be mass-produced for example.
The performance characteristics of on-line control systems, maintenance or the regulation of mechanical systems such as braking or traction systems would be notably improved by the measurement of the stresses exerted on certain of the principal mechanical parts subjected to forces, such as a brake jaw or brake caliper in a braking system for example. Indeed, knowledge of the strains makes it easy to trace them back to the state in which the system has been subjected to action: for example, a knowledge of the stresses exerted on the above-mentioned brake jaw or brake caliper provides for knowledge of the braking torque that is actually exerted on this jaw or caliper independently, in particular, of the slippages or strains in the wheels. In the latter case, proper knowledge of the braking torque notably improves the performance characteristics of the servocontrol of the braking system.
Numerous other applications also require a reliable measurement of the strains in mechanical parts constituting a given system, whether this system is dynamic (for example rotating) or static as in the case of a mechanical holding device or mechanical reinforcement device for example.
These mechanical strains are caused by stresses that get exerted on the parts. Since the strains are related to the stresses through the laws of the mechanics of materials, references to the "measurement of stress" and to the "measurement of strain" amount to one and the same thing. Hereinafter in the text, these two terms shall be used without distinction. These stresses are due, for example, to the application of stretching, compressive, bending, twisting or shear forces.
It is difficult to securely fasten existing stress sensors to the parts to be checked because they generally require a large number of fastening points. Furthermore, the electrical signals given by the strain gauges that constitute these sensors are generally of a very low level whereas the circuits for shaping these signals are at a distance from these gauges. This distance dictates the circulation of signals of a very low level on non-negligible lengths of cables, subjecting these signals to great risks of disturbance. These sensors are therefore set up only for very specific applications, owing notably to their cost and to the difficulty of implementation.
To overcome these problems, one approach may consist, for example, in making measurements of stress by strain gauges that are directly bonded to the parts to be checked. However, implementing such a solution is a delicate task and its automation is difficult, notably because of certain geometries of parts. Furthermore, the adhesive or bonder must faithfully transmit the stresses exerted on the parts to the gauges. Now, since the surface of these parts is not always perfectly plane, there is a risk that this faithful transmission of the stresses as well as the secure fastening of the gauge will not always be ensured. Furthermore, the problem of the distance of the signal-shaping circuits remains. These circuits amplify the signals that they receive from the strain gauges and deliver the results of the stress measurements in analog or digital form for example.
The aim of the invention is to overcome the above mentioned drawbacks, notably by facilitating the fastening, to mechanical parts to be checked, of means for the measurement of the stresses undergone by these parts.